The present invention relates to improvements in the production of TiO.sub.2 pigment. More particularly, the invention is concerned with an improved process for chloride-route TiO.sub.2 pigment manufacture, which is essentially nitrogen-free.
In the conventional production of TiO.sub.2 pigment via the chloride route, a TiO.sub.2 -containing feedstock such as, for example, rutile, ilmenite or titanoferrous slag, or a mixture thereof, is subjected to a fluidized-bed carbochlorination carried out in a chlorinator at a temperature of about 900.degree.-1100.degree. C. and in the presence of a solid carbonaceous reducing agent such as, for example, finely divided particles of coal, coke or anthracite, to produce TiCl.sub.4 which is thereafter converted to TiO.sub.2 by oxidation. Nitrogen is introduced at various points of the process in the form of air or as pure nitrogen. For instance, the finely divided coke is generally charged by gravity into the chlorinator under a nitrogen atmosphere to prevent its combustion with the otherwise surrounding air; the TiO.sub.2 -bearing material is similarly fed under a nitrogen atmosphere, mainly to prevent the chlorine from escaping the chlorinator. Air may also be introduced with the chlorine into the chlorinator to supply auxiliary heat. As a result, a waste gas is generated which contains, along with CO, CO.sub.2 and other gases, a considerable amount of nitrogen which deters commercial recovery and purification of CO.sub.2.
In the carbochlorination step, to ensure fluidization of the ore bed, a minimum carrier gas velocity of about 0.8 ft/sec needs to be maintained. The fluidizing gas is mainly chlorine which reacts with the metallic components of the ore to form their respective chlorides. The carbochlorination reactions which take place are of the type: EQU MeO.sub.x +mCl.sub.2 +C.fwdarw.MeCl.sub.2m +CO.sub.x
where Me is a metal contained in the TiO.sub.2 -feedstock, x is a number varying between 1 and 2, and m is a number varying between 1 and 5; examples of Me are Ti, Fe, Al, Mn, Si, Cr, V and Zr. Even though these reactions are slightly exothermic, heat losses to atmosphere and to the preheating of incoming raw materials require a fine control of the chlorinator temperature in order to balance the thermal and fluid dynamic equilibrium. Changes in the ore composition or coke quality may cause temperature changes which can de-stabilize the fluidized bed operation. Air, oxygen or nitrogen, or a mixture thereof, is sometimes used to assist the control of the thermal balance.